Add String to beginning of String array

Question

Is it possible to add a string to beginning of String array without iterating the entire array.

Answer 1

This is corrected version of solution proposed by @matteosilv:

String[] myArray= {"hi","hi2"};
List<String> list = new LinkedList<String>(Arrays.asList(myArray));
list.add(0, "h3");
myArray = list.toArray(new String[list.size()]);

Answer 2

String[] myArray= {"hi","hi2"};
List<String> temp = new ArrayList<String>(Arrays.asList(prova));
temp.add(0, "h3");
myArray = temp.toArray(new String[temp.size()]);

Answer 3

To do that, you should use List.

And if you want to particularly use the arrays internal, go for an ArrayList

Answer 4

The only way to do this is to maintain a ring buffer. i.e. you have a counter which remembers where the start is and you move that instead of moving all the entries in the array. This only works because you re-define what the "start" means.

See the source for ArrayDeque which has three fields

   86       /**
   87        * The array in which the elements of the deque are stored.
   88        * The capacity of the deque is the length of this array, which is
   89        * always a power of two. The array is never allowed to become
   90        * full, except transiently within an addX method where it is
   91        * resized (see doubleCapacity) immediately upon becoming full,
   92        * thus avoiding head and tail wrapping around to equal each
   93        * other.  We also guarantee that all array cells not holding
   94        * deque elements are always null.
   95        */
   96       private transient E[] elements;
   97   
   98       /**
   99        * The index of the element at the head of the deque (which is the
  100        * element that would be removed by remove() or pop()); or an
  101        * arbitrary number equal to tail if the deque is empty.
  102        */
  103       private transient int head;
  104   
  105       /**
  106        * The index at which the next element would be added to the tail
  107        * of the deque (via addLast(E), add(E), or push(E)).
  108        */
  109       private transient int tail;

So adding to the start works like this

  224       public void addFirst(E e) {
  225           if (e == null)
  226               throw new NullPointerException();
  227           elements[head = (head - 1) & (elements.length - 1)] = e;
  228           if (head == tail)
  229               doubleCapacity();
  230       }


  312       /**
  313        * @throws NoSuchElementException {@inheritDoc}
  314        */
  315       public E getFirst() {
  316           E x = elements[head];
  317           if (x == null)
  318               throw new NoSuchElementException();
  319           return x;
  320       }

Note: it moves the head rather than shifting all the elements down the array.

Answer 5

If you're already using Guava you can use ObjectArrays::concat to do this:

String[] args = ...;
ObjectArrays.concat("prepended", args);

Answer 6

Best I can manage ...

public static void main(String[] args) {
        String[] s = new String[] { "a", "b", "c" };
        System.out.println(Arrays.toString(prepend(s,"d")));
}

public static String[] prepend(String[] a, String el) {
        String[] c = new String[a.length+1];
        c[0] = el;
        System.arraycopy(a, 0, c, 1, a.length);
        return c;
}